1. Field of the Invention
The present invention relates generally to an amplifier, and more specifically to an electronic circuit for amplifying a signal converted from an optical signal transmitted in burst.
2. Description of the Related Art
It is known in the art to make use of a transimpedance amplifier for amplifying a very small electrical current signal which has been converted from an optical signal using a photoelectric cell such as a photodiode. Such kind of conventional transimpedance amplifier is disclosed in Japanese Laid-open Patent Application No. 9-8563.
Prior to turning to the present invention, it is deemed advantageous to briefly discuss, with reference to FIGS. 1 to 3, the above mentioned conventional amplifier.
The amplifier, generally denoted by reference numeral 10, includes an amplifier section 12 and a control section 14. A photodiode 15 has the cathode thereof coupled to a power supply terminal 16. Reference numeral 18 denotes a stray capacitance between the anode of the diode 15 and the ground. As schematically illustrated, an incoming optical signal 20 includes a plurality of burst signals B1, B2, B3, B4, . . . transmitted at predetermined time intervals over an optical fiber (not shown). The optical signal 20 is then converted, at the photodiode 15, to a corresponding electrical current signal 22 which is applied to the amplifier section 12.
The amplifier section 12 comprises an inverting amplifier 24, a buffer 26, two feedback resistors 28 and 30, a phase compensating capacitor 32, and two N-channel MOS transistors 34 and 36 each of which functions as a switch. Such a MOS transistor is interchangeably referred to as a switch. On the other hand, the control section 14 is comprised of a comparator 40 and a set/reset type flip-flop (hereinafter sometimes referred to as FF) 42.
Referring to FIGS. 2 and 3, the optical signal 20 is shown in more detail. As shown in FIG. 2, each burst signal comprises a preamble which is a pattern of reversals, 101010 . . . , repeated for a predetermined time duration. The preamble signal is used for preparing reception of data which follows the preamble. The first three preamble pulses S1, S2, and S3 are shown in FIG. 3.
Turning back to FIG. 1, the comparator 40 checks to determine if the output (denoted by 44) of the amplifier section 12 crosses a reference voltage 46 applied via a terminal 46a. If the output 44 does not cross the reference voltage 46, the comparator issues a logic level 0. On the contrary, when the output 44 crosses the reference voltage 46, the comparator generates a logic level 1 which sets the FF 42. Thus, the output 43 of the FF 42 assumes a logic level 1 which turns on the switches 34 and 36.
When the switch 34 is turned on, the resistor 30 is put into a feedback loop and thus, the gain of the inverting amplifier 24 is determined by the two resistors coupled in parallel. It is understood that when the switch remains off, the gain of the inverting amplifier 24 is solely determined by the resistor 28.
Transimpedance gain TG1 of the amplifier 24 when the switch 34 remains off is given by EQU TG1=A/(A+1)!.multidot.R.sub.28 ( 1)
On the other hand, transimpedance gain TG2 of the amplifier 24 when the switch 34 is closed, is given by EQU TG2=A/(A+1)!.multidot.(R.sub.28 .times.R.sub.30)/(R.sub.28 +R.sub.30)(2)
where A represents the gain of the amplifier 24, R.sub.28 and R.sub.30 represent respectively the resistance values of the resistors 28 and 30.
A reset signal 50 is usually transmitted with the optical signal in a manner to be positioned immediately before or after each burst signal.
Generally, the input current signal 22 changes over a wide range from 0.1 .mu.A to as large as 100 .mu.A. It is thus necessary to control the gain of the amplifier 10 over such a large input dynamic range. Assuming that the amplifier 10 is required to generate an output voltage whose amplitude has a value between 50 mV and 500 mV in order to assure correct operations of subsequent circuitry. Assuming further that the gain A of the amplifier 24 is 30, and the resistance values of the resistors 28 and 30 are respectively 40 K.OMEGA. and 4.44 K.OMEGA.. Under such assumption, the amplifier 24 has an input dynamic range from 1.29 .mu.A to 129 .mu.A. This means that the lower range is undesirably limited.